How do we know….

I want to right a sermon on how we know we're Christian. It bugs the mess outta me when I hear the whole "Well, I'm a good person......sooooooo.......yea." DOESN'T CUT IT! People seem to come up with so many excuses! "Well, I've never killed anybody....." or how about this one, "I know I'm not perfect, but...." DUH! You always know that whatever comes after that is gonna be complete crap because they had to start it out like that. There are very specific reasons that we are Christians and very clear cut ways you will know that Christ is living in you. I want to study up on this and write a sermon on it.